Field of the Invention
The present invention relates to a photovoltaic cell of an inorganic solid excellent in safety and environmental resistance, based on an operation principle utilizing photoexcited structural change of a metal oxide.
Description of the Related Art
Amid growing awareness of global environmental problems such as exhaustion of fossil fuels and warming associated with increases in carbon dioxide, attention is being given to photovoltaic cells as clean energy sources. While materials for photovoltaic cells fall into three categories which are a silicon system, a compound system, and an organic system, the silicon system is most widely used in terms of resource abundance and cost.
In principle, light is applied to the junction between a p-type semiconductor and an n-type semiconductor, so that electrons are generated by photovoltaic effect, moved unidirectionally by rectification, and extracted out of an electrode, thereby converting light energy into electrical energy.
FIG. 12 is a band diagram for explaining the principle of a silicon photovoltaic cell. There are shown in FIG. 12 a conduction band 58, a valence band 60, and a Fermi level 62 when an n-type semiconductor 76 and a p-type semiconductor 78 form a junction. In the vicinity of the junction, electrons 64 and holes 65 diffuse and become bound together, generating diffusion current, so that the electrons 64 and the holes 65 cancel each other, and a depletion layer 80 of few electrons and holes is formed in the vicinity of the junction. At this time, a positive potential is formed in an n-type semiconductor region, and a negative potential is formed in a p-type semiconductor region.
In this state, upon irradiation with light, i.e., sunlight 36 having energy above a band gap, electron-hole pairs are formed in silicon. The electrons 64 and the holes 65 diffuse in silicon and reach the pn junction. By the electric field of the pn junction, the electrons 64 and the holes 65 are separated and move to the n-type semiconductor region and the p-type semiconductor region, respectively. In this process, excessive electrons gather in the n-type semiconductor region which becomes negatively charged, and the p-type semiconductor region becomes positively charged, so that current flows through the load from the electrode of the p-type semiconductor region to the electrode of the n-type semiconductor region.
A big technical problem of the photovoltaic cell is to improve conversion efficiency. Accordingly, various proposals have conventionally been made.
Structure surfaces of the photovoltaic cell include a BSF (Back Surface Field) type (e.g., see Japanese Patent Application Laid-Open Publication No. 2009-182290 (Patent Document 1) and Japanese Patent Application Laid-Open Publication No. 2007-266488 (Patent Document 2)) for reducing carrier recombination loss by providing an electric field on a back surface and a BSR (Back Surface Reflection) type (e.g., see Japanese Patent Application Laid-Open Publication No. 2000-174304 (Patent Document 3)) for reducing operating temperature by reflecting light of energy below a band gap which reaches a back surface without generating carriers and becomes heat.
Further, a photovoltaic cell having light-absorbing layers made of a chalcopyrite structure semiconductor and having a double graded band gap that in the light-absorbing layers a first semiconductor layer decreases a band gap as the band gap approaches a second semiconductor layer and the second semiconductor layer has a band gap larger than a minimum band gap in the first semiconductor layer is proposed as a photovoltaic cell having a band structure ideal for enhancement of energy conversion efficiency (e.g., see Japanese Patent Laid-Open Publication No. 2007-335792 (Patent Document 4)).
Further, a photovoltaic cell structure in which a light-absorbing layer has a localized level or an intermediate band in a band gap by forming a heterojunction pn junction formed by laminating an n-type semiconductor having a larger band gap than the light-absorbing layer on the light incident side of the p-type light-absorbing layer is also proposed (e.g., see Japanese Patent Application Laid-Open Publication No. 2009-117431 (Patent Document 5).
The improvement of conversion efficiency has conventionally been an important problem of the photovoltaic cell of any type including the silicon type.
Hindrances to the conversion efficiency include transmission loss, quantum loss, electron-hole pair recombination loss, loss caused by an imperfect pn junction, and reflection loss of a photovoltaic cell surface. The transmission loss is caused by the transmission of photons having energy below the band gap. The quantum loss occurs when electron-hole pairs generated by photons having energy above the band gap retain energy corresponding to only the band gap and the rest changes to thermal energy. The electron-hole pair recombination loss is recombination loss at the silicon surface and inside. The loss caused by the imperfect pn junction is also a production problem. The reflection loss of the photovoltaic cell surface is caused by the reflection of part of the sunlight from a transparent electrode surface.
As proposed as means for increasing efficiency in Patent Document 5, the photovoltaic cell structure having the localized level or the intermediate band in the band gap of the light-absorbing layer is a structure for reducing loss as a method for effectively decreasing the band gap. However, the structure is formed by cleaning and/or etching the surface of a p-type ZnTe substrate by an organic solvent, forming a p-type ZnTe1-xOx light-absorbing layer by reacting zinc in a vapor state, tellurium (Te) in a vapor state, and radical oxygen on the surface of the p-type ZnTe substrate by molecular beam epitaxy (MBE), and then laminating an n-type ZnO layer on the p-type ZnTe1-xOx light-absorbing layer by reacting zinc in a vapor state and radical oxygen by molecular beam epitaxy (MBE), which is complicated in terms of structure and production.